A separation method for treatment of a fluid may include a separation method using heating or phase-changing, and a separating method using a filtering membrane. The separating method using a filtering membrane is advantageous in that it can obtain high reliability of water treatment since the water purity required can be easily and stably satisfied by adjusting the size of the pores of a membrane. Furthermore, since the separation method using a filtering membrane does not require a heating process, a membrane can be used with microorganism which is useful for separation process but may be adversely affected by heat.
According to an operation method, a filtering membrane module can be largely classified into a suction type filtering membrane module and an external pressure type filtering membrane module.
The suction type filtering membrane module is submerged into a water tank filled with fluid to be treated. A negative pressure is applied to the inside of the filtering membranes, whereby only fluid passes through the wall of each membrane and solid elements such as impurities and sludge are rejected. This suction type filtering membrane module is advantageous in that the manufacturing cost is relatively low and that the installation and maintenance cost is reduced since a facility for circulating fluid is not required. However, the suction type filtering membrane module has a disadvantage of the limitation on flux per unit period.
In case of the external pressure type filtering membrane module, an external pressure is applied to fluid to be treated so that only fluid passes through the wall of each membrane and solid elements such as impurities and sludge are rejected. Even if the external pressure type filtering membrane module necessarily requires a facility for circulating fluid, a flux per unit period in the external pressure type filtering membrane module is relatively larger than a flux per unit period in the suction type filtering membrane module.
When the fluid in which contaminants including solid elements are suspended is filtered through the use of filtering membrane module, the filtering membrane might be easily contaminated due to the contaminants, thereby causing low water permeability of the filtering membrane. Thus, it is necessary to regularly clean the filtering membrane by removing the contaminants from therefrom.
According to a cleaning purpose, a method for cleaning the contaminated filtering membrane may be largely classified into a maintenance cleaning and a recovery cleaning.
A main purpose of the maintenance cleaning is to maintain good permeation performance of filtering membrane. The maintenance cleaning is mainly performed via physical cleaning such as backwashing process or aeration process during a water treatment or after a temporary stoppage of water treatment. The backwashing process removes impurities from a surface of membrane by causing air or water to flow backward through the membrane during a temporary stoppage of water treatment. The aeration process removes impurities from a surface of membrane by generating rising air bubbles through air jetted from an aeration pipe positioned under the membrane.
The recovery cleaning is performed when the filtering membrane module exhibits serious deterioration in permeation performance of a membrane due to contaminants accumulated by a long-term use. A main purpose of the recovery cleaning is to recover permeation performance of the membrane. The recovery cleaning is to clean the filtering membrane through the use of chemical cleaning agent after stopping the fluid-filtering process.
One method of the recovery cleanings according to the related art is using hot water together with the chemical cleaning agent. Generally, according as a surface temperature of the filtering membrane is raised more, cleaning efficiency becomes better. Thus, the hot water is used for enhancing the cleaning efficiency of the recovery cleaning. For using the hot water, a heating apparatus has to be additionally provided, whereby economical efficiency is lowered due to the increased power consumption.
Another method of the recovery cleanings according to the related art is using only the chemical cleaning agent without using the hot water. Generally, an organic material largely exists in the contaminants. If using an alkaline cleaning agent, efficiency of removing the organic material can be enhanced. Thus, instead of using the hot water, the strong-alkaline cleaning agent with pH 12 is used for enhancing the efficiency of removing the organic material. However, the strong-alkaline cleaning agent might cause a damage of the filtering membrane. Especially, polyvinylidene fluoride (PVDF) typically used for the filtering membrane is weak in strong-alkaline material. Thus, if carrying out the recovery cleaning using the strong-alkaline cleaning agent, the cleaning efficiency is raised somewhat, but the filtering membrane may be damaged.